Thick film gold conductor compositions have been developed commercially for high adhesion strength fine line resolution microcircuit conductors, chip and wire bonding, electrical terminal contact metallization, and etchable pastes for thermal printer head and microwave applications. Precious metals other than gold are also used in thick film pastes but gold is specified for the high performance high reliability hybrid microcircuits used in military, medical, aerospace and instrumentation applications. To achieve high performance and reliability, reproducibility is essential and control of particle size and shape in the production of the gold powder is necessary.
There are many chemical methods of producing gold powders and each one may include variations involving pH, dilution, and temperature among others. A frequently used technique is to precipitate gold from chloroauric acid solution. This may be accomplished by a variety of active metals such as zinc, magnesium, iron, etc., by inorganic reducing agents such as ferrous sulfate, sodium sulfite, sulfur dioxide, and hydrogen peroxide, or organic reducing agents such as formic acid, formaldehyde, or others. However, these processes can not be easily controlled and therefore are not found to produce materials with uniform particle size and shape, both of which are very important in the improvement in the properties of thick film pastes.
The characteristics of the gold powder such as surface area, particle size, particle shape, etc. are dependent on the conditions from which the powder was made. Physical characteristics such as these influence the chemical processability and determine to a large extent the appearance, usefulness, and efficiency of the gold powder in the particular application.
With increasing circuit complexity and decreasing feature sizes, thick film pastes must be composed of pure and chemically uniform particles with very well controlled particle size distributions. Spray pyrolysis has emerged as a promising technique for the production of phase--pure powders with compositional homogeneity at the molecular level.